This invention relates to a conditioned air distribution system and in particular to a velocity controller for controlling the volume flow of conditioned air in a variable air volume control system.
A conventional air distribution system that has been often used in the air conditioning industry is a constant volume air conditioning system in which a velocity controller maintains a constant velocity of discharge air from the outlet of a duct. In the constant volume system the velocity controller is on line all of the time. The controller both limits the maximum velocity and also maintains a constant velocity of discharge air.
In this constant volume type of system the primary concern is the control of air flow velocity at one setting, and the temperature regulation is obtained by mixing hot air with cold air to obtain the desired air temperature at the discharge of the duct.
In this kind of system any zone in a building measures the same amount of discharge air, whether heating or cooling, and the amount of the discharge air is the amount which the constant velocity controller is set for.
Under many conditions of operation the constant volume air conditioning system can be wasteful of energy. For example, in many cases, the full amount of the discharged air volume may not be needed for heating or cooling. And the required mixing of hot air and cold air can also be inefficient in many instances.
Because of recent pressures to economize and to conserve energy for ecology reasons, the air conditioning industry has become quite interested in variable air volume systems.
In variable air volume systems the concern is the control of velocity from a minimum (or no air flow) to a maximum amount of air flow, and the amount of air flow is varied in relation to the heating or cooling requirements of the room.
For example, assuming that the room is being cooled by cold air from the duct, as the temperature in the room goes up (as indicated by a room thermostat signal), a greater amount (volume) of cold air is discharged from the duct to cool the room back to the desired temperature; and as the room temperature goes down, a lesser amount (volume) of cold air is discharged from the duct to permit the room temperature to rise back to the desired level.
In this variable air volume system the amount of air is regulated in response to room requirements; and under most conditions of operation, there is no mixing of hot air with cold air to provide variations in the temperature of the air discharged from the duct -- as is the case in constant air volume systems.
The variable air volume systems are therefore inherently more energy efficient than constant air volume systems.
Some prior art variable air volume systems have used a flow controller in which the room thermostat controlled, or positioned, the actuator for the flow control valve in the duct until the air flow velocity in the duct exceeded the setting on a velocity controller. In such a system the velocity controller only acted as a limiter, and was basically a manual adjustment for a set point. Thus, any time the thermostat is positioning the actuator and the air flow through the duct is less than that set at the maximum velocity limit (as set by the velocity controller) there is no control of the flow velocity of the air flowing through the duct. This system works satisfactorily if the static pressure does not vary. But if the static pressure of the air flow in the duct does vary (that is, if the static pressure of the duct increases or decreases with no change in the room temperature or the position of the flow control valve in the duct), the volume of air discharged from the duct decreases or increases with that static change.
In practice, static air pressure changes in the order of one inch water column to six inch water column can occur as a result of the varying air flow in system when other parts of the system are being opened or closed.
Thus, the room temperature could change in response to static pressure in the duct and did not change solely in response to changes in the room load. With a change in static air pressure in the duct, but no change in the room air temperature, the prior art system could therefore flow a different amount of cold air through the duct until the thermostat repositioned the actuator and the flow control valve in the duct; and this was an unstable loop.
A more desirable control system is one in which the room thermostat works in conjunction with the velocity controller to maintain a constantly regulated amount of air into the room in proportion to the room thermostat's demands, and independently of variations in the static pressure in the duct. In this system, if the static pressure in the duct increases (with a resultant increase in velocity pressure) the velocity controller senses the change and cuts back and regulates the air flow velocity in the duct to the same velocity as it was before the increase in the static pressure. The same is true if the static air pressure decreases. The air flow velocity controller then senses the decrease in velocity and repositions the actuator to maintain the correct velocity.
This more desirable system requires that the velocity controller be reset by the room thermostat, and it is a primary object of the present invention to interlock the room temperature (as indicated by a thermostat signal) with the velocity set point of a velocity controller in a variable air volume control system so that the velocity controller maintains a constantly regulated amount of air into a room in proportion to the room thermostat's demands and independently of variations of static pressure in the duct.
Another problem that is presented in variable air volume systems is the problem of control offset when operating at very low static pressures (very low velocity pressures). A full time velocity controller must necessarily operate during some conditions of operation with low velocity pressures in the duct. And the problems that are presented in controlling air flow at low velocity pressures are quite different from the control parameters that are presented in a velocity controller which is used only as a limiting device for limiting the maximum velocity. In a limiting device which limits the maximum velocity, the velocity controller is always working with very high static pressures; and control offset is an insignificant factor at high velocity pressures.
In a full time velocity controller which is reset by a thermostat, as noted above, as the minimum setting or zero velocity setting is approached, any change in static pressure in the duct (with the resultant offset in the set point of the controller) greatly affects the velocity of the air in the duct.
Thus, even though the velocity controller is constructed to interlock the room thermostat with the velocity controller to provide a set relationship independent of static pressure in the duct (and to maintain a certain velocity for any demand in the room thermostat), as the air flow velocity approaches zero the change of static pressure and resultant offset of the controller can make the controller ineffective to provide the required air flow demanded by the room load at low air flow rates in the duct.
It is therefore another important object of the present invention to automatically compensate for control offset caused by changes in static pressure at low air flows in the duct.